mtd: nand: raw: convert nand_dt_init() to ofnode_xx() interface

nand_dt_init() is still using fdtdec_xx() interface.
If OF_LIVE flag is enabled, dt property can't be get anymore.
Updating all fdtdec_xx() interface to ofnode_xx() to solve this issue.

For doing this, node parameter type must be ofnode.

First idea was to convert "node" parameter to ofnode type inside
nand_dt_init() using offset_to_ofnode(node). But offset_to_ofnode()
is not bijective, in case OF_LIVE flag is enabled, it performs an assert().

So, this leads to update nand_chip struct flash_node field from int to
ofnode and to update all nand_dt_init() callers.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>